Due to these intriguing characteristics and the importance of leaves for plant performance and function, many aspects of leaf development have been extensively studied. Curiously, leaf shape is often largely conserved between related species with genetic variations in thousands of genes, while a single mutation can sometimes induce morphological differences similar to those that distinguish species and even families (e.g., Barkoulas et al., 2008). On the other hand however, the process is so plastic that environmental factors can affect mature leaf size by an order of magnitude. On the one hand it is so strictly regulated that the resultant leaf morphology is a reliable characteristic for taxonomic classification. The development of leaves in dicotyledonous plant species is an intriguing process, resulting from a complex interplay of a multitude of regulatory pathways. It forms the basis for crop yield, turn-over in ecosystems and the means for the plant to adapt to environmental conditions and experimental treatments. Understanding the regulation of plant growth and its constituent organs is an important objective in biology. We discuss to what extent the knowledge available in the literature is suitable to be applied in systems biology approaches to model the process of leaf growth, in order to better understand and predict leaf growth starting with the model species Arabidopsis thaliana. We focus on the molecular networks that control exit from stem cell fate, leaf initiation, polarity, cytoplasmic growth, cell division, endoreduplication, transition between division and expansion, expansion and differentiation and their regulation by intercellular signaling molecules, including plant hormones, sugars, peptides, proteins, and microRNAs. To build the basis for such understanding, we review current knowledge on the spatial and temporal regulation mechanisms operating on cells, contributing to the formation of a leaf. Therefore, to fundamentally understand leaf development, one needs to be able to reconstruct the developmental pathway of individual cells (and their progeny) from the stem cell niche to their final position in the mature leaf.

Cells are both the basic building blocks of the leaf and the regulatory units that integrate genetic and environmental information into the developmental program. In order to improve crops for higher productivity and resistance for future climate scenarios, it is important to obtain a mechanistic understanding of leaf growth and development and the effect of genetic and environmental factors on the process. Through its photosynthetic capacity the leaf provides the basis for growth of the whole plant. 2Department of Mathematics and Computer Science, University of Antwerp, Antwerp, Belgium.1Laboratory for Molecular Plant Physiology and Biotechnology, Department of Biology, University of Antwerp, Antwerp, Belgium.Shweta Kalve 1, Dirk De Vos 1,2 and Gerrit T.

Leaf primordia